This invention relates to improvements in angular position sensors, and in particular to a rotary encoder suitable for determining the absolute angular position of the rotor of a brushless electrical motor over a complete mechanical revolution.
Angular position sensors—often referred to as rotary encoders—have a wide variety of applications although they are most often used to determine the angular position of the rotor of a motor. This positional information can be used in a control strategy for the motor. In many applications-including motors-the rotating shaft or rotor will make an almost unlimited number of revolutions during their working life. To allow for this most encoders are non-contact and employ an encoding mask fixed to the shaft which is sandwiched between at least one detector and a light source. The encoding mask carries at least one annular track of encoding elements which pass between the source and the detector to modulate the radiation reaching the detector.
Rotary encoders generally fall into one of two categories: absolute position encoders and relative position encoders. An absolute position encoder is a device capable of providing an output signal indicative of the angular position of the rotor to a limited angular resolution. These devices typically include more than one encoder tracks and the output is commonly an N-bit signal (N greater than one) with each bit or “channel” being obtained from a respective track on the encoder. The choice of pattern of the encoding elements in each track determines the value or “state” of the output signal at any angular position of the encoder. For example, to provide an output signal with 16 unique states requires 4 encoder tracks. Each output state corresponds to a range of angular positions of the rotor over a complete 360 degree revolution. In the example given with a 4-bit (16 state) output each state indicates that the rotor is within a particular 22.5-degree range (360/16 degrees). A key advantage of such a device is that a position value is obtained upon the instant that the device is switched on from the pattern of encoder elements between the source and the detector.
Relative encoders—sometimes referred to as incremental encoders, on the other hand, detect the position of the encoder disk relative to a datum by counting transitions between states, which occur as encoder elements pass between the source and the detector. In its purest form only a single track of encoder elements is required. The output will have only two states, and will repeat many times over one revolution. These devices typically offer a higher resolution than absolute position devices and are less expensive. A disadvantage over absolute encoders is that the position information is lost if power is removed from the counter and the encoder is rotated whilst power is switched off. When power is returned the position of the datum is unknown.
The most common form of incremental encoder is the quadrature encoder. This employs an encoding disk, which carries a single annular track of encoding elements. When rotated past two detectors the elements produce two quadrature (often sinusoidal) output signals that each vary periodically between one state and another over a revolution. One signal A leads the other B by ninety degrees. This type of device, which is well known in the art, has a relatively high resolution determined by the period of the encoding elements. It also allows the direction of travel of the rotor to be determined.
The applicant has proposed to use the output of an angular position sensor to provide an indication of the electrical angle of a multipolar motor. This can be used in order to control the commutation of the motor. For example, with a p=6 pole motor there are p/2=3 complete electrical revolutions per mechanical revolution of the rotor. The sequence of commutation of the motor windings is therefore repeated 3 times within each complete mechanical revolution. For motor control only the electrical commutation states need to be determined and an absolute encoder can therefore be provided which has an encoding pattern that repeats three times over a mechanical revolution. However, if an absolute measurement of mechanical position is required it becomes necessary to provide additional encoding tracks. For the example of a six pole motor a minimum of an additional two tracks are required to give sufficient values to uniquely identify all 18 commutation states. The need for the extra tracks simply to determine which electrical revolution the motor occupies is disadvantageous as it increases the complexity of the device.